A technology is known that uses surface plasmon resonance (SPR) in a biosensor that detects DNA, proteins, sugar chains or the like, and a chemical substance sensor that detects metal ions, organic molecules or the like.
This technology employs a chip having such a structure as a glass plate is coated with a noble metal (gold, silver or the like) by vapor deposition, with the side of the glass opposite to the side coated with the metal put into contact with an optical prism via a refractive index matching oil, wherein a laser beam or white light is introduced through the prism into the glass and the intensity of the reflected light is measured.
The incident light irradiates the glass under a total reflection condition, and SPR occurs at a particular incidence angle due to an evanescent wave at the surface coated with the metal on the side opposite to the side where the light is incident.
When SPR occurs, the evanescent wave is absorbed by the surface plasmon, and therefore intensity of the reflected light decreases significantly around the incidence angle.
The intensity of reflected light at the incidence angle that induces SPR and at an angle near the incidence angle that induces SPR varies depending on the thickness and the relative dielectric constant of a material deposited on the metal surface. A conventional SPR sensor makes use of this effect to determine the extent of coupling (film thickness or weight) of the specimen under investigation, by modifying a material that couples with or is adsorbed on the specimen on the surface of a metal and detecting the change in the incidence angle or reflectivity caused by the coupling or adsorption of the specimen in the vicinity of the metal surface.
Applications of the technology based on SPR include those disclosed in Patent Document 1 “Optical sensor, detection method using optical sensor and formation of molecular recognizing film for optical”, Patent Document 2 “Sensor utilizing Attenuated total reflection”, Patent Document 3 “Optical waveguide type SPR measurement chip, method for manufacturing the same and SPR measuring method”, Patent Document 4 “Waveguide structure, its manufacturing method, and surface Plasmon resonance sensor and refractive index change measurement method using the waveguide structure”, and Patent Document 5 “Optical waveguide type surface plasmon resonance sensor and optical waveguide type surface plasmon resonance device”.
However, the prior art technologies that utilize the surface plasmon resonance described above have the problem that the sensitivity is not enough to detect a small specimen.
An attempt to overcome this drawback by using an optical system similar to the SPR sensor has been reported (Non-Patent Document 1), according to which molecules adsorbed on a sensor surface can be measured with high sensitivity by forming an optical waveguide on the surface of a noble metal of the SPR sensor and utilizing an optical waveguide mode excited in the optical waveguide.
The optical waveguide mode is caused by multiple reflections within a dielectric material. FIG. 1 shows a substrate structure of a chip that demonstrates an optical waveguide mode.
Light incident on a glass at an angle passes through the glass and illuminates a reflector layer so as to generate an evanescent wave on the side of a dielectric optical waveguide. When the evanescent wave couples with the optical waveguide mode in the dielectric optical waveguide, a part or all of the incident light propagates in the dielectric optical waveguide and is therefore not reflected. Thus coupling of the incident light with the optical waveguide mode in the dielectric optical waveguide causes a decrease in the intensity of reflected light. This decrease in the intensity of the reflected light occurs only at incidence angles near a particular angle for the light of a given wavelength.
The particular incidence angle and the intensity of reflected light at the incidence angle depend greatly on the relative dielectric constant of the surface of the dielectric optical waveguide. As a result, when a substance is adsorbed, deposits or otherwise attaches to the surface of the dielectric optical waveguide, the incidence angle and the intensity of reflected light change. The conventional optical waveguide mode sensor measures this change to determine the presence of a particular substance and the quantity of the substance.
Sensitivity of the optical waveguide mode sensor can be improved by increasing the surface area of the optical waveguide. Non-Patent Document 1 describes that high-sensitivity sensing can be achieved by using alumina that is formed by anodic oxidation as the optical waveguide. However, it is difficult to control the size of pores formed in alumina, and alumina is not stable in the presence of acid or alkali. Moreover, it may not be easy to modify a substance that couples with or adsorbs a specimen on the surface thereof.
Patent Document 1: Japanese Unexamined Patent Application, First Publication No. 6-58873
Patent Document 2: Japanese Unexamined Patent Application, First Publication No. 2002-195942
Patent Document 3: Japanese Unexamined Patent Application, Application No. 2000-339895
Patent Document 4: Japanese Unexamined Patent Application, First Publication No. 2004-170095
Patent Document 5: Japanese Unexamined Patent Application, First Publication No. 2004-184381
Non-Patent Document 1: Journal of Physical Chemistry B Vol. 108, pp. 10,812-10,818, 2004